Five barges sit to the east of the Vicksburg Tallulah Regional Airport at dusk waiting for traffic to open along the Mississippi River near Vicksburg, Miss. A barge carrying thousands of gallons of oil struck a railroad bridge and began leaking before dawn Sunday. The accident forced the closure of a 16-mile stretch of the lower Mississippi, a major inland corridor for vessels carrying oil, fuel, grain and other goods. (ASSOCIATED PRESS / The Vicksburg Evening Post, Melanie Thortis )

VICKSBURG, Miss. (AP) — The Coast Guard said Tuesday that about 7,000 gallons of crude oil were unaccounted for aboard a leaking barge that had rammed a railroad bridge near Vicksburg on the Mississippi River, which remained closed for a third day as crews slowly pumped out oil.

Petty Officer 3rd Class Jonathan Lally said it’s not clear that all of the 7,000 gallons leaked into the river since the collision early Sunday. Some of it, he said, could have seeped into void spaces inside the barge.

Lally said a plan to pump oil from the stricken barge onto another barge — a process known as lightering — had been approved but it was unclear how long that would take or when the river might reopen to vessels. He said the other barge was in route.

Environmental impact, Lally said, has been minimal because a boom is containing the leak around the barge and the leak is slow.

On Tuesday, tugs were pinning the ruptured barge to the bank on the Louisiana side of the river, across from Vicksburg’s Riverwalk and Lady Luck casinos. Their engines churned the muddy water for which the Mississippi is so famous. A few workers could be seen walking on top of the stricken barge.

An orange boom bobbed in the water just downstream and another boom was set up as a second line of defense to keep the oil from spreading.

The economic impact is another matter.

At least 54 vessels, including towboats and barges, were idled Tuesday on the closed 16-mile stretch of one of the nation’s vital commerce routes.

Vessel traffic tends to be less in January than during peak harvest season, when grain from the U.S. heartland is shipped south to be loaded onto massive ships near New Orleans.

“The Coast Guard advised our hazardous materials unit that the river would be closed indefinitely to all traffic,” Lt. Julie Lewis of the Louisiana State Police said.

About 168.4 million tons of cargo a year moves along the Mississippi River between Baton Rouge, La., and the mouth of the Ohio River, carried by nearly 22,300 cargo ships and 162,700 barges, according to the Army Corps of Engineers. About 3.6 million tons of cargo is handled annually by the port of Vicksburg.

When low water threatened to close the river earlier this month, the tow industry trade group American Waterways Operators estimated that 7.2 million tons of commodities worth $2.8 billion might be sidelined over the last three weeks of January.

Salt destined for Northern roads is moving upriver in January, said spokeswoman Ann McCulloch. “We’re still moving corn, soybeans and grain, but also coal and petroleum ... stone, sand and gravel,” she said Tuesday.

Barges carry 20 percent of the nation’s coal and more than 60 percent of its grain exports, according to the group.

Crews have been working around the clock to contain and remove oil since the barge, owned by Corpus Christi, Texas-based Third Coast Towing LLC, struck a railroad bridge and began leaking early Sunday. The company has refused to comment on the incident.

Severe weather expected to sweep through the area late Tuesday and on Wednesday could shut down operations for a time, authorities said.

Ron Zornes, director of corporate operations for Canal Barge Co. of New Orleans, said each idled towboat could cost a company anywhere from $10,000 to $100,000 a day. The low end would be for a single boat with a couple of barges and the high end for one in “a system of towboats that acts sort of like a bus system.”

“So if one bus is stopped it gums up the whole system,” he said.

Lally said crews had skimmed 1,596 gallons of an oil-and-water mixture from the river.

Nature’s Way Marine LLC of Theodore, Ala., has been named the responsible party for the oil spill, a designation that is assigned under the federal Oil Pollution Act.

The barges were being pushed by the company’s tug Nature’s Way Endeavor when the collision happened. The company has declined requests for information from the Associated Press, referring calls to the Coast Guard.

Companies found responsible for oil spills face civil penalties tied to the amount of oil that spilled into the environment. Lally said it’s too early in the investigation to know whether the company could face penalties or fines because of the collision.

The Nature’s Way Endeavor was pushing two tank barges when the collision with the bridge happened about 1:30 a.m. Sunday, authorities said. Both barges were damaged, but only one leaked. Authorities declared the bridge safe after an inspection.

The leaking tank, which was pierced above the water line, was carrying 80,000 gallons of light crude, authorities said. The Coast Guard hasn’t said how much oil was in the other tanks on the barge.

Lally said there was no evidence that oil was washing ashore.

“We did have a Coast Guard helicopter crew fly for 60 miles up the shoreline there to see if they could spot any environmental impact and they weren’t able to find anything,” he said, adding that a Coast Guard boat surveying 15 miles south of the site also detected nothing.

Engineers familiar with the river near Vicksburg say sharp bends and fast currents make it difficult to navigate.

The Coast Guard says it’s too early in the investigation to say what caused the barges to bump the bridge on Sunday.

Estimates of the leak at Vicksburg came the same day that a federal judge in New Orleans approved a government settlement of criminal charges with BP PLC — and $4 billion in penalties — stemming from the company’s 2010 oil spill in the Gulf of Mexico. In that spill, BP’s well blew for months, spewing more than 200 million gallons of crude oil.